They must be using a bathroom for both mares and stallions, or maybe Hitch needed to go so badly he could wait to run all the way to the stallions room. Could also be that this is the stallions room and the mares room was full, whichever bathroom they were in and weather the other bathroom was full, the toilets they sit on now will soon be full after these desperate ponies relive themselves in them.
In the end, you can use your imagination on what really happened that lead to this image. I would love to hear what you think in the comments!